A multiple antenna system is classified as an open-loop system in which channel information is not informed to a transmitting terminal, and a closed-loop system in which the channel information is informed to the transmitting terminal and the transmitting terminal uses the channel information to precode a symbol and to transmit the precoded symbol.
In the open-loop system, a transmission symbol is space-time encoded such that the symbol may obtain a high diversity gain and an encoding gain. However, such a space-time encoding method has been developed to be optimized in a statistical channel, and therefore instantaneous channel variations may not be appropriately applied.
The closed-loop system obtains instantaneous channel information from the transmitting terminal, and uses the instantaneous channel information to perform a precoding operation so that a high quality symbol may be transmitted. Since the symbol is transmitted according to instantaneous channel situations, an instantaneous channel facing problem may be efficiently solved, and an array gain may be obtained by using multiple antennas. An optimized combination transmitting method and an own-beam-forming method have been known as examples of the closed-loop system, and it has been widely known through various research that maximum diversity and array gains may be obtained in the multiple antenna system by using the above methods.
The closed-loop multiple antenna system has excellent performance as described above, but there are some problems involved in realizing it. Since channels of transmitting and receiving antennas are not identical in a conventional communication system, it is actually impossible to obtain instantaneous downlink information at the transmitting antenna. Therefore, channel information estimated by the receiving terminal is required to be transmitted back to the transmitter as feedback information so that the transmitting antenna may obtain the channel information. However, it is impossible to transmit the instantaneous channel information in a limited feedback channel band environment. Accordingly, a quantization precoding method for precoding the symbol at the transmitting terminal by using the channel information has been proposed.
A quantization precoder uniformly quantizes a space formed by the channels of the multiple antennas to form a codebook (hereinafter referred to as a “quantization codebook”), and transmits channel information to the transmitting terminal by using an index of the codebook since the codebook is shared by the transmitting and receiving terminals. When the size of the codebook is sufficiently great, the quantization precoding method may obtain performance close to the optimum combination of transmitting and own-beam-forming methods in which the exact information is informed to the transmitting terminal. When assuming that channel elements of the multiple antennas are iid complex Gaussian random variables, N (the size of the codebook) unitary matrixes representing a total random unitary matrix space is obtained by using characteristics of a right matrix of a channel matrix having uniform distribution on the random unitary matrix, and the N obtained unitary matrixes are used as the quantization codebook.
As shown in FIG. 1, in a transmitting terminal 20 of the multiple antenna system, a channel estimator 21 estimates a channel by using a training symbol transmitted from a transmitting terminal 10, and transmits information on the estimated channel to a codeword selector 24. The codeword selector 24 selects a codeword for minimizing a bit error rate of a current channel among codewords of the quantization codebook 25 based on the estimated channel information, and transmits an index of the selected codeword to the transmitting terminal 10 through a feedback channel.
In the transmitting terminal 10, a modulator 11 modulates an inputted data stream, and a quantization precoder 12 precodes the modulated data stream and transmits it to the receiving terminal 20 through a multiple antenna channel 30. When the index of the codeword transmitted by the receiving terminal 20 is i, the quantization precoder 12 precodes the modulated data stream by using Wi as a precode matrix and transmits the precoded data stream to the receiving terminal 20. A signal vector y received at the receiving terminal 20 is given as Math Figure 1.y=HWis+n  [Math Figure 1]
Here, H denotes a channel matrix having an MR×MT dimension, in which respective elements of the matrix follow iid complex Gaussian distribution having an average of 0 and a variance of 1. MR and MT respectively denote the numbers of the receiving antennas and the transmitting antennas. Wi denotes an ith codeword having an MT×Ns dimension, in which all column vectors of the matrix have an orthonormal characteristic with each other. s denotes a symbol vector having an Ns×1 dimension, which satisfies E{ssH}=εs/Ns. n denotes a noise vector having an MR×1 dimension, in which respective elements follow a complex Gaussian distribution having an average of 0 and a variance of N0.
In addition, when the receiving terminal 20 uses a zero forcing (ZF) linear receiver, an effective signal to noise ratio (SNR) of a kth stream may be given as Math Figure 2.
                                          η            k                    =                                    ρ                              M                T                                      ⁢                          1                                                [                                                            {                                                                                                    (                                                          HW                              i                                                        )                                                    H                                                ·                                                  (                                                      HW                            i                                                    )                                                                    }                                                              -                      1                                                        ]                                                  k                  ,                  k                  ,                                                                    ,                  k          =          1                ,        …        ⁢                                  ,                  N          s                                    [                  Math          ⁢                                          ⁢          Figure          ⁢                                          ⁢          2                ]            
Here, ρ=εs/N0 denotes an SNR of the multiple antenna system, and [·]k,k denotes a (k,k)th element of a matrix. Since ρ and MT among the induced effective data stream are used in common for all the data streams, a metric for selecting the codeword is given as Math Figure 3 after eliminating the common values ρ and MT, which will be referred to as an SNR metric.
                                          η            ~                    k                =                  1                                    [                                                {                                                                                    (                                                  HW                          i                                                )                                            H                                        ⁢                                          (                                              HW                        i                                            )                                                        }                                                  -                  1                                            ]                                      k              ,              k                                                          [                  Math          ⁢                                          ⁢          Figure          ⁢                                          ⁢          3                ]            
When the SNR metric for each data stream is induced, a codeword minimizing a bit error rate is selected as shown in Math Figure 4.
                              i          *                =                  arg          ⁢                                          ⁢                                    max              i                        ⁢                          {                                                min                                                            k                      =                      1                                        ,                                                                                  ⁢                    …                    ⁢                                                                                  ,                                          N                      s                                                                      ⁢                                                      η                    ~                                    k                                            }                                                          [                  Math          ⁢                                          ⁢          Figure          ⁢                                          ⁢          4                ]            
A codeword index i* selected by using Math Figure 4 is transmitted to the transmitting terminal 10 through a feedback channel 40, and the quantization precoder 12 of the transmitting terminal 10 precodes the data stream modulated by the modulator 11 by using Wi* as the precode matrix and transmits the precoded data stream through the transmitting antenna. As described, the linear receiver 22 of the receiving terminal 20 uses Math Figure 5 to detect a symbol from among the data stream transmitted after being precoded by Wi* at the transmitting terminal 10 detects a symbol.ŝ=Zy=(HWi*)†y  [Math Figure 5]
Here, (·)† denotes a pseudo inverse operation of a matrix.
However, the quantization precoder 12 has limitations regarding instantaneous or statistical channel variations since the data stream is fixed in the quantization precoder 12. That is, the quantization codebook used in the quantization precoder 12 is designed while assuming independent coefficients of multiple spatial channels. Therefore, performance is steeply reduced when a channel spatial correlation exists and the number of channel ranks is less than the number of initially determined streams due to the channel spatial correlation. Since such a channel rank deficiency may be instantaneously caused in a spatially independent channel, the performance is considerably deteriorated in the quantization precoder 12 having the fixed number of the data streams.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.